Beef refrigeration and preservation method

ABSTRACT

Fresh beef is both refrigerated and preserved during transit by providing a liquid mixture comprising 7-15 mol percent oxygen and 85-93 mol percent nitrogen in a storage container, and spraying same into an enclosed chamber to surround the beef with cold gas maintained at 30-40* F.

United States Paten Moline Mar. 14, 1972 [54] BEEF REFRIGERATION AND 3,102,777 9/1963 Bedrosian et a1. ..21/58 PRESERVATION METHOD 3,102,779 9/1963 Brody et a1. ..21/58 3,313,631 4/1967 Jenson ..21/58 [72] whte Hams 3,365,307 1/1968 Dixon ..21/58 [73] Assignee: Union Carbide Corporation, New York, 3,368,873 2/1968 Fuller et a1 ..2l/58 NY. 3,508,881 4/1970 Hagenauer et all. .....2l/58 [22] Filed: Aug 8, 1969 3,287,925 11/1966 Kane et a1 ..62/64 [21] Appl. No.2 848,540 Primary Examiner-Meyer Perlin Assistant Examiner-Ronald C. Capossela U-S. CL Attorney-Paul Rose, Thomas OBrien, John Le Fever .g/IM 59/197, 62/5 l4, and Lawrence G. Kasmner [51] Int. Cl ..F25d 17/02 58 Field of Search ..62/62, 64, 7s, 5 14; 99/194, 1 ABSTRACT 99/5 189; 21/58 Fresh beef is both refrigerated and preserved during transit by providing a liquid mixture comprising 715 mol percent ox- [56] References cued ygen and 85-93 mol percent nitrogen in a storage container, UNITED STATES PATENTS and spraying same into an enclosed chamber to surround the beef with cold gas maintained at 30-40 F. 2,834,188 5/1958 Bradford. .62/62 3,100,971 8/1963 Morrison ..62/64 6 Claims, 9 Drawing Figures PATENTEDMAR 14 I972 SHEET 1 OF 7 PATENTEDHAR 14 1972 3, 54 ,474

sum 2 BF 7 I .INVENTOR SHELDON W. MOLINE ATTORNEY PATENTEDMAR 14 m2 SHEET 5 OF 7 STORAGE TIME,DAYS

HNVENTOR SHELDON W. MOLINE ATTORNEY PATENTEUMAR 14 I872 3, 4 ,474

SHEET 6 UF 7 1 48 72 STORAGE TIME (DAYS) FIG? doaanivaadwai LVHW .NVE TOR SHELDON W. MOLINE ATTORNEY PMENTEDMAR 14 I972 SHEET 7 [IF 7 ho) beef warm beef 0 deep probe 5 shallow probe O O 0 O O O O 9 8 7 6 5 4 3 b TIME (DAYS) FIGIQ.

INVENTOR SHELDON W. MOLINE ATTORN EY BEEF REFRIGERATION AND PRESERVATION METHOD BACKGROUND OF THE INVENTION This invention relates to a method for in-transit refrigeration and preservation of fresh beef by a low-boiling liquefied oxygen-containing gas.

The in-transit refrigeration of perishables such as fresh beef by spraying cold fluid from a liquid nitrogen storage body into the product storage chamber is well-known, as exemplified by the system described in Kane et al. US. Pat. No. 3,287,925. Special problems are encountered when the shipped beef is to be kept in the refrigerated but unfrozen state for relatively long periods, e.g., 24-30 hours or more. Railroad or truck shipment periods are frequently of this duration when the beef is shipped from a slaughter house to a distant retail market area.

Unless the storage chamber is frequently opened to the atmosphere during the in-transit period, the periodic introduction of cold nitrogen gas (to maintain a predetermined low temperature range) results in a high nitrogen purity gas environment. A slight positive pressure of nitrogen gas is often maintained to prevent inleak of warm air.

It has been discovered that when fresh beef is exposed to a nitrogen atmosphere for periods of at least 24 hours, the beef outer surface may acquire a grey or purple color and sometimes even a slippery texture. Surprisingly this surface condition occurs even at low temperatures of 30-40 F., and is unique to fresh beef. That is, fresh pork does not assume the same surface condition when exposed to the same environment and temperature for the same duration. Also, the surface discoloration does not result when fresh beef is exposed to the aforedescribed cold nitrogen environment for appreciably shorter periods, e.g., less than about 24 hours.

The color of fresh beef is determined by the chemical state of the muscle pigment, myoglobin (Mb). This complex heme protein contains an iron-porphyrin group and can bind oxygen in a loose and readily reversible combination in which the iron remains in the reduced (ferrous) state. Although the iron atom in Mb combines with oxygen and retains its reduced state, the oxygen for which Mb is a carrier also tends to convert the pigment to the oxidized (ferric) state. In the presence of oxygen, Mb is converted into two different pigments, Met Mb (brown) and Oxy Mb (red), the oxidized and oxygenated fonns, respectively. The relative portions of these two forms depend upon the partial pressure of oxygen, the formation of Met Mb being favored at low oxygen partial pressure.

At all oxygen pressures there is a constant conversion of Mb or Oxy Mb to Met Mb, but as a result of enzymatic oxidation of substrates, particularly glucose, there is a continual supply of reducing substances capable of reducing the Met Mb. In fresh beef held in air, the bright red color of Oxy Mb is observed on the surface (where there is both a plentiful supply of oxygen and reducing substances). In the interior, however, there is essentially no oxygen and the Mb is in the reduced state which has a typical dark purple color. As long as a supply of reducing compounds exists in the tissue, the pigment remains in the reduced state (as either Mb or Oxy Mb). As soon as these compounds are consumed, the iron of the Mb is oxidized to brown Met Mb. This apparently occurs when beef is held in a nitrogen atmosphere for long periods.

It is emphasized that the previously described discoloration of the fresh beef outer surface is frequently only temporary, depending on the extent and duration of the Met Mb conversion. When such beef is reexposed to the normal oxygen partial pressure of air, it often gradually blooms," i.e., returns to a red color. However, the shipped beef is often sold to a retailer virtually on arrival at its destination, and even a temporary unattractive condition may drastically limit its marketability.

Another important consideration in the in-transit refrigeration of fresh beef is that the refrigerated beef is preferably not frequently exposed to warm moisture-laden air. This is because excessive moisture tends to condense on the beef and contribute to a slippery appearance. In long-haul, i.e., over 400 miles shipment of refrigerated beef, this is usually not a problem because the access doors to the product chamber are not frequently opened. That is, the observed slippery texture is not due to frequent exposure to warm moisture-laden air. However, it should also be understood that an improved beef refrigeration method must not be characterized by frequent exposure to warm moisture-laden air. On the other hand the beef must not lose its original moisture, i.e., be dried, by the refrigeration method.

Still another requirement of an improved in-transit longhaul refrigeration method for fresh beef is that the environment should not be susceptible to oxygen enrichment, i.e., sufficient to support rapid combustion.

A further requirement of an improved in-transit long-haul refrigeration method for fresh beef is that it should be simple and reliable. That is, the method should be susceptible of practice with rugged apparatus which does not require close balancing of flows due to frequent intransit jarring, and which requires a minimum of valuable on-board space which could otherwise be used for beef storage.

It is an object of this invention to provide an improved method for long-haul intransit refrigeration of fresh beef.

Another object of this invention is to provide an improved long-haul intransit cryogenic spray refrigeration method for fresh beef which provides moist (but not slippery) red beef at the point of destination without loss of moisture.

A further object is to provide an improved method which can be practiced with compact, simple apparatus not requiring careful balancing of flows.

Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.

SUMMARY This invention is an improvement to the prior art method for intransit spray refrigeration of fresh beef in a storage chamber by passing low-boiling refrigerant from a liquid container through longitudinally spaced openings in a spray conduit overhead the fresh beef, in response to sensing of the chamber gas temperature. In particular, a liquid oxygennitrogen mixture is provided in the container comprising 7-15 mol percent oxygen and -93 mol percent nitrogen. The refrigerant mixture is periodically sprayed into the chamber so as to surround the beef with oxygen-nitrogen gas and maintain the chamber gas temperature at 30-40 F.

In a preferred embodiment of this method, a liquid oxygennitrogen mixture comprising 10-13 mol percent oxygen and 87-90 mol percent nitrogen is charged in the container, and fresh beef having core temperature below 60 F. is loaded in the chamber. After isolating the chamber from the atmosphere for example by closing access doors, the liquid oxygen-nitrogen mixture is dispensed from the container and sprayed into the chamber so as to surround the beef with oxygen-nitrogen gas and lower the chamber gas temperature to a predetermined range of 30-34 F. Thereafter additional cold oxygen-nitrogen mixture is periodically sprayed into the chamber to maintain the chamber gas temperature within the predetermined range.

The aforementioned liquid mixture comprises at least seven mol percent oxygen to avoid irreversible conversion of fresh beef surface pigment to Met Mb during; sustained exposure of the beef to cold nitrogen-rich gas. More than 15 mol percent oxygen does not further improve blooming of fresh beef and is undesirable because it may become enriched above 21 percent oxygen and thus cause an increased flammability hazard. The range of 10-13 mol percent oxygen and 87-90 mol percent nitrogen is a preferred balance of these characteristics.

The oxygen-nitrogen cold gas environment temperature is maintained above about 30 F. to avoid moisture freezing in the beef, and below about 40 F. to retard bacterial growth and also because the pigment conversion to Met Mb is appreciably accelerated at higher temperatures. A temperature range of 30-34 F. is preferred from these standpoints.

This invention has been successfully employed to refrigerate and preserve fresh beef for periods of at least 8-10 days. That is, at the end of this period the beef has not suffered irreversible color change, regains its bloom on exposure to air, is not slippery and judged completely acceptable by commercial standards. Moreover, this improvement is achieved in a system which precludes the possibility of oxygen enrichment above 21 percent with accompanying increased flammability hazard, and employs virtually the same simple compact apparatus used by the prior art for pure nitrogen spray refrigeration of fresh beef. Other advantages of the invention will be apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a schematic view taken in cross-sectional elevation of a truck semi-trailer incorporating apparatus suitable for practicing one embodiment of the invention.

FIG. 2 is a schematic view taken in cross-sectional elevation of a railroad car incorporating apparatus suitable for practicing another embodiment wherein a fan is used to circulate the cold oxygen-nitrogen refrigerant gas.

FIG. 3 is an isometric view of a floor vaporizer construction especially suitable for use in the FIG. 2 apparatus. I

FIG. 4 is a graph showing the effect of oxygen concentration and temperature on the color value of longissimus dorsi (rib eye) beef when stored in cold nitrogen refrigerant gas for periods up to five days.

FiG. 5 is a graph showing the effect of oxygen concentration and temperature on the color value of psoas major (tenderloin) beef.

FIG. 6 is a graph showing the effect of temperature on aerobic bacterial growth on longissimus dorsi in 13 percent 87 percent N gas for periods up to five days.

FIG. 7 is a graph showing the cooling rate of hot beef when exposed to oxygen-nitrogen gas at 34 F.

FIG. 8 is a graph showing the cooling rate of hot beef when exposed to oxygen-nitrogen gas at 38 F., and

FIG. 9 is a graph showing the cooling rate of hot and warm beef when exposed to oxygen-nitrogen gas at 30 F.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, FIG. 1 illustrates apparatus in which a truck semi-trailer body constitutes thermally insulated storage chamber 11 for fresh beef 12 often in the form of hanging carcasses. This chamber may be of standard construction for typical mobile refrigerated chambers, e.g., reinforced aluminum siding outer walls, plywood paneled inner walls and plastic foam insulating material between the two walls. The chamber is closed to the atmosphere but need not be airtight, as access means such as rear doors (not illustrated) are needed for insertion and removal of the fresh beef 12.

A double-walled thermally insulated container 13 is associated with storage chamber 11 for storing pressurized lowboiling liquefied oxygen-nitrogen mixture. The construction of such containers is well-known and is, for example, depicted in Loveday et al. U.S. Pat. No. 2,951,348. Container 13 (either cylindrical or rectangular) is depicted within storage chamber 11 but also may be positioned outside this chamber. Container 13 includes an outer shell completely surrounding an inner storage vessel to form an evacuable insulation space therebetween. This space is preferably filled with an efficient solid thermal insulating material, as for example alternate layers of radiation-impervious barriers such as aluminum foil separated by low conductive fibrous sheeting, as for example glass fibers. This particular highly efficient insulation is described in U.S. Pat. No. 3,007,596 to L. C. Matsch. Other suitable insulating materials include layers of aluminum coated-polyethylene terephthalate. Alternatively, powdered insulating material, as for example perlite or finely divided silica, may be employed.

To remove gases accumulating in the evacuated insulating space, an adsorbent material as for example calcium zeolite A,

or a gettering material as for example powdered barium, may be provided therein to retain a high level of insulating quality.

The liquid oxygen-nitrogen mixture comprising 7-15 volume percent oxygen may for example be provided directly from a low temperature air separation plant. Alternatively it may be obtained by sequentially charging storage container 13 with appropriate quantities of high purity nitrogen and oxygen. The vessel within storage container 13 is filled with liquid oxygen-nitrogen mixture by means well known to the prior art (or sequentially with pure liquids), such as for example connecting a source of cold liquid stored at above atmospheric pressure to the container. If the cold liquid is stored at a pressure below the operating pressure of container 13, a suitable pump would be employed and usually additional heat would be added to the pressurized liquid before transferring it into container 13. The cold liquid is preferably charged into container 13 and stored therein at saturated conditions and at temperatures corresponding to a vapor pressure above 10 p.s.i.g. with the entire liquid and vapor substantially in equilibrium. If the aforementioned highly efficient insulation is used, there is no appreciable amount of heat inleak to the inner storage vessel of container 13 and the stored cold liquid is dispensed only by this as-charged vapor pressure. Altematively the cold liquid may be charged to container 13 under non-saturated conditions and even in the subcooled state. Under these circumstances, it would probably be necessary to provide means for building sufficient internal pressure on demand to discharge the liquid. Those skilled in the art will appreciate that this heat may be introduced externally, using the well-known pressure building coil. The latter includes a liquid discharge conduit, an atmospheric heat vaporizer and a conduit for returning the resulting vapor to the container gas space (not illustrated). As still another variation known to the art, a less efficient heat insulating material may be used so that sufficient atmospheric heat inleak is available to vaporize sufficient stored liquid refrigerant to form gas pressure to insure liquid discharge on demand.

It is preferred to store the liquid oxygen-nitrogen refrigerant at pressure below about p.s.i.g., because higher pressures require impractically small spray conduit openings. Moreover, the inherent lag characteristics of commonly used temperature sensing elements would make adequate control of the liquid refrigerant withdrawal more difficult. The storage pressure is preferably above about 10 p.s.i.g. to provide sufficient driving force for substantially uniform distribution of cold fluid through the overhead spray openings; Storage container pressures of 15-22 p.s.i.g. have been found quite satisfactory.

Refrigerant discharge conduit 14 is joined at one end to liquid storage container 13 and has control valve 15 therein. The other end of refrigerant discharge conduit 14 joins overhead spray conduit 16 and extends substantially the entire length of product storage chamber 11, with openings 17 spaced along the length for discharging refrigerant fluid therein. Openings 17 may be oriented either horizontally or slightly downwardly in the circumference of conduit 16, and preferably have a circular configuration for unifonn symmetrical discharge of sprayed refrigerant. Thermal insulation may be provided around the outer surface of spray conduit 16 if desired. As will be described hereinafter in detail, the oxygennitrogen refrigerant may be sprayed into chamber 11 as small liquid droplets which are immediately vaporized. Alternatively the refrigerant may be at least partially vaporized in discharge conduit 14 and sprayed into chamber 11 as a liquidvapor mixture or entirely in the vapor form.

Refrigerant fluid flow control means are provided including a temperature sensing element 18 such as a bulb positioned within the storage chamber 11 gas space. This bulb 18 is connected by signal receiving means 19 to temperature controller 20, and signal transmitting means 21 provides communication between the controller 20 and control valve 15 in fluid discharge conduit 14. The flow control means may be electrically or pneumatically operated.

In normal operation, after the access doors are closed cold oxygen-nitrogen fluid, normally liquid, is intermittently dispensed from liquid refrigerant storage container 13 through conduit 14 and control valve 15 in response to signals from temperature bulb 18. Controller is set to maintain the gas temperature within a predetermined range, e.g., 30-40 F. That is, if the sensed temperature rises above the set point, controller 20 responds by sending a signal through transmitting means 21 to open valve 15 and permit refrigerant flow therethrough until the sensed temperature drops below the set point. Controller 20 then responds by closing valve 115 and terminating refrigerant flow until the chamber gas temperature again rises above the set point temperature. To avoid over pressurization of beef storage chamber 11, part of the oxygennitrogen gas may be discharged through vent 22 to the environment outside chamber 111.

The apparatus of FIG. 2 is especially suited for practicing an embodiment of the instant method which permit closer temperature control and minimizes end-to-end and top-to-bottom temperature variations within chamber 111. The specific system of FIG. 2, however, is not part of this invention, but is described in greater detail and claimed in copending application Ser. No. 777,643 filed Nov. 21, 1968, now abandoned, in the name of Lester K. Eigenbrod.

It has been previously indicated that the practice of this invention requires the chamber to be maintained at 30-40 F. This is the chamber gas temperature as for example sensed by bulb l8, and the beef itself will be slightly warmer. By way of example, if the sensed gas temperature is 30 F. and the beef has stored in chamber 11 for sufficient duration for cooling to approximate equilibrium, the beef outer surface may be at about 32 F. and the beef core slightly Wanner, e.g., 34 F. The beef may be kept at 32 F. without freeze damage because the moisture is in a semi-combined form which does not freeze at this level.

In the FIG. 2 apparatus, refrigerant liquid discharge conduit 25 branches from refrigerant fluid discharge conduit 14 and is provided with control valve 26 therein. The other end of liquid discharge conduit 25 is joined to liquid vaporizing heat exchanger 27 positioned in the bottom portion of beef storage chamber 1 1 and extending substantially the length thereof. Heat exchanger 27 preferably comprises at least two passes, the first section 27a extending to the opposite end of chamber 11 and the second section 27b returning to the chamber end near refrigerant liquid storage container 13.

Conduit sections 27a and 27b are surrounded by thermal insulation 28 preferably having sufficient effectiveness to control heat conductance to 5-30 B.t.u. per hour per foot conduit length, as described in the aforementioned copending application Ser. No. 777,643, now abandoned. Thermally conductive metal floor means 29 is contiguously associated with and in heat transfer relation to thermal insulation 28. Metal floor means 29 extend at least part of the length and at least part of the width of the product storage chamber beneath the conduit 27 thermal insulation 28 assembly, and is illustrated as a flat plate. Product platform 29a may be superimposed on the heat exchanger 27 thermal insulation 28 metal floor member 29 assembly, and beef 12 may be positioned thereon in boxes or as carcasses. It is usually preferred to hang fresh beef 12 as illustrated in FIG. 1 for more uniform refrigeration. Separate passageways 270 are usually provided for end-to-end flow of the circulating oxygen-nitrogen gas mixture in heat exchange relation with the vaporizing refrigerant liquid and beef 12, as discussed hereinafter in detail.

The floor vaporizer outlet 30 joins gas expander 31 positioned in the upper portion of chamber 111 beneath spray conduit l6 and preferably near one end thereof. Expander 31 may for example be a commercially available sliding vane-type air motor with an inlet pressure of about 10-25 p.s.i.g. Operating at ZOO-1,500 r.p.m. or greater, but a turbine type expander may be used instead. Expander 31 is joined by shaft coupling means 32 to fan 33 so that the energy released by expansion of the pressurized gas from higher to lower superatmospheric pressure is recovered as rotational energy to drive the fan. The latter serves to improve circulation of gas within chamber 11 and reduce end-to-end temperature differences. If desired, the vaporized refrigerant gas may be further warmed before passing through expander 31 by flow through a superheater (not illustrated) outside chamber ll 1.

Fan 33 circulates refrigerant gas dispensed from overhead openings 17 to the opposite end of chamber 111 and to the lower end thereof. The gas then flows through passageways 270 in heat exchange relation with the vaporizing refrigerant liquid in conduits 27a and 27b, and fresh beef 12. The warmed chamber environment gas emerging from the opposite end of passageway 27c flows upwardly behind bulkhead 34 and is at least partially recirculated by fan 33. The latter also insures circulation of the expanded gas discharged from expander SI through port 35. If desired, a portion. of the recirculating gas may be discharged to the atmosphere through vent 22.

Fan 33 may alternatively be electrically driven from a power source, such as a battery and generator unit (not illustrated). For such electrically-driven fans, the gas from the floor vaporizer outlet is usually dispensed in the vicinity of the fan for effective circulation. The advantage of the gas-driven fan for the FIG. 2 apparatus is that external power is not required. The energy to operate fan 33 is derived from the on-board pressurized refrigerant fluid itself.

Another feature of the FIG. 2 apparatus is a second temperature responsive control system for refrigerant liquid in discharge conduit 25 from container 113. Temperature sensing element 36, as for example a thermostat bulb, is positioned in the bottom portion of chamber 1111 in thermal association with a passageway 27c for the circulating; environment gas. This element may be located in the passageway gas space or against the passageway wall. The element 36 is connected by signal receiving means 37 to temperature controller 38, and signal transmitting means 39 provides communication between the controller and control valve 26 in liquid discharge conduit 25. The flow control means may be electrically or pneumatically operated. The temperature response range of element 36 is also between 30 and 40 F., preferably 23 F. colder than the temperature setting for overhead bulb 18.

In normal operation of the FIG. 2 system, oxygen-nitrogen refrigerant liquid is continuously dispensed from storage container 13 through discharge conduit 25 and control valve 26 to floor vaporizer 27. This liquid is at least vaporized therein by surrounding heat, i.e., from the product chamber environment gas, from the fresh beef product itself through the solid conductive heat transfer path joining the product, and from heat inleak through the floor from the environment surrounding chamber 11. Under normal conditions the vaporized liquid is at least partially superheated in floor vaporizer 27 so that the gas released through outlet 35 is at an intermediate temperature between the liquid and the product chamber, e.g., about -l0 F. to 15 F. The remaining sensible refrigeration of this gas is at least partly recovered by fan circulation along with the cold spray dispensed through openings 17.

FIG. 3 illustrates a preferred floor vaporizer 27 construction suitable for use in the FIG. 2 railcar embodiment, including multiple metal channels 41 positioned in the bottom of the storage chamber llll from end-to-end thereof. The FIG. 3 construction is also suitable for practicing the invention in highway trucks and semi-trailers. A first group of channels 41 are open at opposite ends and form the passageways 270 for circulating chamber environment gas. It should be appreciated, however, that the channel-type construction may also be employed for heat exchangers 27 not including gas passageways 27c. The refrigerant liquid vaporizing conduits 27a are positioned within a second group of channels 41 and thermal insulation 28 substantially fills the space between the conduit outer wall and the channel wall. Superheating conduits 27b are similarly positioned within another second group of channels 41 and second thermal insulation 28 substantially fills the space between the conduit outer wall and the channel wall.

TABLE II Initial O2 in temp.. Chamber chamber Chamber beef core gas temp. gas (mol gas R.H. Beet initial Beet color Test number F.) F.) percent) (percent) color atter3 days 90 30 01-02 90 Red. Purple. 90 40 0.2-0.8 90 Purple-red" Do. 76 30 0.1-0.2 90 Purple Do. 90 40 0.4-1.4 90 ..do-. Do. 88 34 1.5-3.0 90 Purple-red Purple-red. 70 34 8. -100 90 do Do. 96 34 8.0-10.0 90 do D0. 86 34 90 .do Red. 80 38 7 90 urple Red. 86 38 10 90 Purple-red Red. 55 40 0.8-7.0 90 do Purple. 95 40 0.8-7.0 90 do Do.

s 1 Relative humidi y- The advantages of this invention were demonstrated in a series of tests in which various cuts of fresh beef were stored in various oxygen-nitrogen environments at different temperature levels. United States Department of Agriculture graded choice sides were secured 4 to 72 hours after kill for these tests. Internal meat (core) temperatures upon arrival at the test facility were 55-90 F., in some instances the meat having been cooled from body temperature in the conventional manner by circulating cold air. Strip loins were removed from carcasses and cut into several equal sections. Each had a longissimus dorsi (rib eye) and psoas major (tenderloin) muscle. The beef chamber for the first series of tests was the body section of a refrigeration truck and piping very similar to FIG. 1 was used. The chamber internal dimensions were 84 inches wide X 162 inches long X 72 inches high, and oxygen concentrations were determined by a conventional analyzer. The gas environmental and beef core temperatures were continuously recorded and relative humidity was measured but not controlled. The beef was hung from an overhanging rail. After placing the beef in the chamber, the desired test gas was flushed through the chamber to bring the oxygen content to the desired level. The test gas was sprayed into the chamber in response to a temperature sensing so as to maintain a predetermined temperature range.

In order to evaluate beef quality, a variety of subjective and objective tests were used including the following: reflectance spectra, color, pl-l, rancidity, flavor ratings, bacterial analyses and aroma. Samples were examined initially and after storage at regular intervals. Probably the most informative tests were those relating to color. Color determinations were made by comparing meat color with Munsell color chips (Munsell Color Company, Baltimore, Maryland) which evaluate color in terms of hue, value, and chroma. As a result of variations between meat and chip color, this match was limited to the best approximation.

The standard color values for meat are summarized in Table I and the data from this first series of tests is summarized in Table ll. To evaluate this data it should be understood that beef having a color rating of at least 8 (purple-red) will regain its bloom on exposure to air with no permanent discolorization. However, with lower color ratings below 8 there may be a permanent loss of bloom although beef having color ratings of at least 5 is commercially acceptable.

Inspection of Table ll reveals that in Test Nos. 1-4 the oxygen' concentrations in the nitrogen-rich gas were exceedingly low (0.1-1.4 mol percent 0,) and the purple color after three days indicated a permanent loss of bloom It should be especially noted that this occurred at relatively low temperatures of 30-40 F. where the aerobic bacterial growth rate is desirably low and one might expect that there would be relatively little pigment oxidation to the undesired brown Met Mb. Test Nos. 11 and 12 are characterized by higher oxygen concentrations than Tests 1-4 (0.8-7.0 mol percent 0,) but on the average below the lower limit of this method, i.e., seven mol percent. Again the purple color indicated irreversible discolorization and considerable surface oxidation to Met Mb.

In contrast, Test No. 6-10 demonstrate the superior results attainable by this invention in the sense that the beef color after three days was either the highly desirable red (Tests Nos. 8-10) or the reversible purple-red (Tests Nos. 6 and 7). Test No. 9 is particularly significant as demonstrating that the red color can be obtained in a 7 percent oxygen-93 percent nitrogen atmosphere at 38 F., starting with beef having a purple surface color. Test No. 5 appears somewhat inconsistent with Tests Nos. 1-4 in that the beef color did not deteriorate from the original purple-red, even though the oxygen concentration was only 1.5-3.0 mol percent. The probable explanation is that the relative humidity of the Test N0. 5 chamber was higher than in Tests Nos. 1-4.

In another group of tests a four foot X two foot X two foot plastic chamber was used to simulate various environments during intransit refrigeration of fresh beef. Provisions were made for flushing the chamber at the rate of one liter/hour with oxygen-nitrogen mixture. The control meat samples were exposed to circulating air. Relative humidity was regulated by using glycerol solutions of various ratios placed in shallow pans within the test and control environments. In order to control the relative humidity in the test environment, the oxygennitrogen gas was bubbled through a glycerol solution prior to introduction in the chamber. ln these particular tests the relative humidity was maintained at 70 percent.

FIGS. 4 and 5 are graphs summarizing data from these tests showing the effects of environment temperature and oxygen concentration on the color of two different beef cuts (longissimus dorsi and psoas major) for period up to five days. Comparing the 13 percent 0 and 21 percent 0, tests at 37 F., it is apparent that the beef color at the end of the storage period was very similar. However, the 13 percent 0 -87 percent N environment is advantageous in avoiding the possibility of oxygen enrichment and increased flammability hazard.

Comparing the 37 F. and 42 F. tests at 13 percent 0,, there is a remarkable and unexpected difference in beef color after sustained storage. For example, FIG. 4 indicates that after five days storage at 37 F., longissimus dorsi still has the maximum rating of 10 whereas the sample at 42 F. has a color rating of only 3 which indicates an irreversible gray and a commercially unacceptable product. Similarly, FIG. 5 reveals th ie LzL h' QqwtQrat'satlilitrssasmonths; as l r value of 5 and is acceptable whereas the 42 F. sample has a color value of only 2 which is irreversibly black and unacceptable.

I sv ma i s qata Oaths bacterial s wn fi sls s s- 9 simus dorsi samples in a preferred environment composition of this invention (13 percent 87 percent N but at temperature according to the invention (37 F.) and at above the 30-40 F. range, i.e., 42 F. Accordingly it isolates the effect of temperature on the beef deterioration rate since the number of bacteria is an indicator of the accumulation of slime on the outer surface. Standard swabbing procedures American Public Health Association, 1958) were used to estimate the bacterial population. Aerobic and anaerobic organisms were cultured using Tryptone glucose yeast extract agar and thioglycollate agar. It will be immediately apparent from FIG. 6 that the bacterial count after four-five days of storage is unexpectedly several orders of magnitude higher at 42 F. than at 37 F., thereby demonstrating the importance of the 30-40 F. range in achieving the improved results of this method. g g l g FIGS. 7 and 8 summarize still another series of tests which demonstrate a further advantage of the preferred embodiment wherein the oxygen-nitrogen gas surrounding the beef is maintained at 30-34 F. In these tests, the round sections of beef hindquarters were placed in the same truck chamber as used in the Table II tests and cooled in 34 F. gas (FIG. 7) and 38 F. (FIG. 8). Temperature was continuously monitored by three probes at depths of one inch (shallow), three inches (medium) and six inches (deep). Inspection of the curves reveals a remarkable difference in cooling rates even though the gas differed only by four degrees. By way of example, the 34 F. gas provided 38 F. temperature at the deep level in 48 hours (FIG. 7) whereas the 38 F. gas required 72 hours to cool the beef deep level to the same 38 F., i.e., 50 percent longer time. This difference is very significant because the rate of meat discoloration is temperature dependent and the faster cooldown rate substantially reduces the amount of Met Mb Although this invention may be practiced with beef supplied at any temperature, it is preferably cooled to a core (deep) temperature below about 60 F. by conventional means before exposure to the cold oxygen-nitrogen gas. This is because the beef may be cooled to essentially the gas temperature in a much shorter period and surface discoloration minimized as previously discussed. This was demonstrated in a further series of tests wherein the beef was supplied at core temperatures of 60 F. (warm) and 93 F. (hot) and cooled in 30 F. oxygen-nitrogen gas. By way of example, after one day the initially warm beef has cooled to about 35 F. but the initially hot beef was still at 50 F.

Summarizing, fresh beef may be stored for relatively long periods on the order of five days without permanent discolorization and slippery texture, by envelopment in a cold gas comprising 7-15 mol percent oxygen and -93 mol percent nitrogen which is maintained at 30-40 F.

Although preferred embodiments of the invention have been described in detail, it is contemplated that modifications of the method may be made and that some features may be employed without others, all within the spirit and scope of the invention.

What is claimed is:

1. In a method for intransit spray refrigeration of fresh beef in a storage chamber by passing low-boiling refrigerant from a liquid container through longitudinal spaced openings in a spray conduit overhead said fresh beef in response to sensing of the chamber gas temperature, the improvement of: providing a liquid oxygen-nitrogen mixture comprising 7-15 mol percent oxygen and 85-93 mol percent nitrogen in said container, and periodically spraying the refrigerant mixture into said chamber so as to surround said beef with oxygen-nitrogen gas and maintain said chamber gas temperature at 30-40 F.

2. A method according to claim ll wherein the relative humidity of said chamber gas is above percent.

3. A method according to claim 1 wherein said liquid oxygen-nitrogen mixture comprises 10-13 mol percent oxygen and 87-90 mol percent nitrogen.

4. A method according to claim 11 wherein said chamber gas temperature is maintained at 30-34 F.

5. A method according to claim 1 wherein said fresh beef is loaded into said chamber at core temperature below 60 F.

6. In a method for intransit spray refrigeration of fresh beef in a storage chamber by passing low-boiling refrigerant from a liquid container through longitudinally spaced openings in a spray conduit overhead said fresh beef in response to sensing of the chamber gas temperature, the improvement of: charging a liquid oxygen-nitrogen mixture comprising l0-l3 mol percent oxygen and 87-90 mol percent nitrogen in said container; loading fresh beef having core temperature below 60 F. in said chamber and Isolating the chamber from the atmosphere; dispensing said liquid oxygen-nitrogen mixture from said container and spraying same into said chamber so as to surround said beef with oxygen-nitrogen gas and lower said chamber gas temperature to a predetermined range of 30-34 F.; and thereafter periodically spraying additional cold oxygen-nitrogen mixture into said chamber to maintain said chamber gas temperature within said predetermined range.

i l= =F l 

2. A method according to claim 1 wherein the relative humidity of said chamber gas is above 90 percent.
 3. A method according to claim 1 wherein said liquid oxygen-nitrogen mixture comprises 10-13 mol percent oxygen and 87-90 mol percent nitrogen.
 4. A method according to claim 1 wherein said chamber gas temperature is maintained at 30-34* F.
 5. A method according to claim 1 wherein said fresh beef is loaded into said chamber at core temperature below 60* F.
 6. In a method for intransit spray refrigeration of fresh beef in a storage chamber by passing low-boiling refrigerant from a liquid container through longitudinally spaced openings in a spray conduit overhead said fresh beef in response to sensing of the chamber gas temperature, the improvement of: charging a liquid oxygen-nitrogen mixture comprising 10-13 mol percent oxygen and 87-90 mol percent nitrogen in said container; loading fresh beef having core temperature below 60* F. in said chamber and isolating the chamber from the atmosphere; dispensing said liquid oxygen-nitrogen mixture from said container And spraying same into said chamber so as to surround said beef with oxygen-nitrogen gas and lower said chamber gas temperature to a predetermined range of 30-34* F.; and thereafter periodically spraying additional cold oxygen-nitrogen mixture into said chamber to maintain said chamber gas temperature within said predetermined range. 